Fails to find BEL documents that were renamed

[cthoyt]

After renaming Lombardo2015.bel to lombardo2015.bel (among other changes) in this Travis CI build, the following error was raised:

  File "/home/travis/virtualenv/python3.6.3/bin/pybel-git", line 11, in <module>
    sys.exit(main())
  File "/home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/pybel_git/cli.py", line 43, in ci
    graph = from_path(file_name, manager=manager)
  File "/home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/pybel/io/lines.py", line 71, in from_path
    with codecs.open(os.path.expanduser(path), encoding=encoding) as file:
  File "/home/travis/virtualenv/python3.6.3/lib/python3.6/codecs.py", line 895, in open
    file = builtins.open(filename, mode, buffering)
FileNotFoundError: [Errno 2] No such file or directory: 'bel/Lombardo2015.bel'